The Mekong is Southeast Asia's most storied river. Its rich biodiversity though has been under threat for decades from things like overfishing and overpopulation. Environmentalists are now adding dams to that list. Towards the headwaters of the Mekong is Laos, one of the poorest nations in the world. It's now planning a series of dams to generate hydropower and income. But environmentalists and neighboring countries, like Cambodia and Vietnam, are crying foul. They say the dams might have a devastating impact on the Mekong basin. Michael Sullivan reports.

MICHAEL SULLIVAN, BYLINE: OK, it's 9:00 in the morning. I am out in the middle of the Mekong. Cambodia is on my left, Laos on my right and dead ahead, I'm looking at two, maybe three dolphins - Irrawaddy dolphins - who are frolicking in this pool that's just below the area called Si Pan Done. And that's basically where the Mekong splits into seven or eight different channels and then comes back together here. Now, about three quarters of a mile up this channel up here - that's where they're going to put this proposed Don Sahong dam. And that's the one environmentalists say if they do put it in, it's going to have a devastating impact on the fish population here and may mean the end of the dolphin population as well. We chug up the channel in question - past fishermen in tiny boats - to see the proposed Don Sahong site. It's a fast-moving section of water maybe 50 yards wide and my first thought is - what's the big deal? Why does damming just one channel of the river here scare so many people? Turns out, it's the only one that works - for fish anyway.

AME TRANDEM: The Hou Sahong channel is the only channel that fish are able to migrate up and downstream on a year-round basis. Other channels all have obstacles - waterfalls or man-made structures that are catching fish.

SULLIVAN: That's Ame Trandem with the environmental group International Rivers. She worries not only about the fish, but the people who catch them, and so do they. Almost everyone here fishes for a living. One local, who doesn't want to be named, isn't optimistic.

UNIDENTIFIED FISHERMAN: We export the fish in this river every season. So if they have the hydro-powered dam all the people - no more jobs. No more fishing.

SULLIVAN: You think there will be no more fish left after the dam is built?

FISHERMAN: Yeah. No - no more fishing after the dam.

SULLIVAN: What would happen if you spoke out against the dam, if you told the government - we don't want the dam here - what would happen?

FISHERMAN: We have nothing against. If they do thing, they do, then we follow them.

SULLIVAN: You don't get to say no?

FISHERMAN: No, we cannot say no.

SULLIVAN: He makes a slashing gesture across his throat, a reminder that Laos is a one-party communist state that brooks no opposition. Peter Hawkins is the environmental manager for the Don Sahong project, and one of the few people from that side willing to talk about it. He says the concerns of locals and the environmentalists are valid, but he also says they've been dealt with.

PETER HAWKINS: I'm confident that the mitigation measures we can employ here will allow fish to pass the barrier that we're going to create.

SULLIVAN: Hawkins says his company, Megafirst, is building fish passageways around the proposed site - and if that doesn't solve the problem?

HAWKINS: We have the opportunity - if we do not have a 100 percent success rate in terms of passage, we can continue to improve those bypasses. So we see this as a work in progress.

SULLIVAN: There's another work in progress a few hundred miles upstream that environmentalists fear even more, the Xayaburi dam, which the government says is about 30 percent completed. Unlike the Don Sahong, this one will block the entire river.

JIAN-HUA MENG: We do see that Laos has every right to develop on its own pathway and should not be controlled by outside people telling them what to do. Maybe in terms of the Mekong mainstream, they have been listening to the wrong advisors.

SULLIVAN: That's Jian-hua Meng, a dams and hydropower specialist for the World Wildlife Fund. He says the Xayaburi dam is being built without any real knowledge of the downstream effects. Water, Meng says, punishes every mistake you make, especially when it comes to allowing fish to migrate up and down the river.

MENG: The effectiveness of such fish passage mechanisms is quite well-proven for European or North American rivers, but at the Mekong, we don't have five fish species which we have to take care of - we have 70 - maybe even more. And we have no clue about it.

SULLIVAN: International River's Ame Tramden says fisheries experts estimate at least 43 species of fish are likely to go extinct because of the impact of the dam, including the Mekong giant catfish – the world's largest. The other problem is sedimentation - dirt, silt the river carries downstream to Cambodia and Vietnam. The Xayaburi, Trandem says, will have major food security implications as well.

TRANDEM: By blocking sediment where there's a lot of agricultural productivity and rice growing, these areas are going to suffer a lot because they're no longer getting the same nutrients. And so this will have significant impact.

SULLIVAN: Vietnam and Cambodia aren't happy about either dam. They want work on both suspended while further study is conducted about the long-term effects they may have. Laos has politely told both and everyone else - sorry, but we're going ahead. For NPR News, I'm Michael Sullivan. It's MORNING EDITION from NPR News.

NPR transcripts are created on a rush deadline by a contractor for NPR, and accuracy and availability may vary. This text may not be in its final form and may be updated or revised in the future. Please be aware that the authoritative record of NPR’s programming is the audio.